Time Synchronization System and Time Synchronization Method

1. A time synchronization system for transmitting and receiving a pulse signal at a constant interval at a synchronization timing between first and second communication apparatuses connected through a first optical fiber and a second optical fiber, which are two-core bidirectional, to synchronize time, the time synchronization system comprising: the first communication apparatus that includes a first transmitter configured to transmit a first pulse signal of a first wavelength and a second pulse signal of a second wavelength different from the first wavelength to the second communication apparatus through the first optical fiber, and a first receiver configured to receive a plurality of pulse signals including the first pulse signal and the second pulse signal transmitted from the second communication apparatus through the second optical fiber; and the second communication apparatus that includes a second receiver configured to receive the first pulse signal and the second pulse signal from the first optical fiber, and a second transmitter configured to generate a third pulse signal of a wavelength identical to the second wavelength of the second pulse signal when receiving the first pulse signal, to simultaneously transmit the first pulse signal and the third pulse signal to the first communication apparatus through the second optical fiber, to generate a fourth pulse signal of a wavelength identical to the first wavelength of the first pulse signal when receiving the second pulse signal, and to simultaneously transmit the second pulse signal and the fourth pulse signal to the first communication apparatus through the second optical fiber, wherein the first communication apparatus further includes a calculating unit, including one or more processors, configured to calculate a propagation delay amount in the first optical fiber from a first proportional relationship between a first propagation delay time difference between the first pulse signal and the fourth pulse signal of the identical first wavelength received by the first receiver and a second propagation delay time difference between the first pulse signal of the first wavelength and the third pulse signal of the second wavelength that is different from the first wavelength, and a second proportional relationship between a round-trip delay time in the first and second optical fibers and the propagation delay amount in the first optical fiber, and a control unit, including one or more processors, configured to control the first pulse signal and the second pulse signal to be transmitted from the first transmitter with a time difference corresponding to a difference between current and last values of the calculated propagation delay amounts such that the difference is zero.

2. The time synchronization system according to claim 1 , wherein the control unit performs a first control of the first pulse signal and the second pulse signal to be simultaneously transmitted to the first optical fiber at a beginning of transmitting and receiving the first and second pulse signals between the first and second communication apparatuses, and performs a second control of the first pulse signal and the second pulse signal to be transmitted with a time difference corresponding to a difference between the propagation delay amount in the first optical fiber calculated by the calculating unit according to the first control, and a predetermined propagation delay amount, serving as a reference, in the first optical fiber such that the difference is zero.

3. A time synchronization method by a time synchronization system for transmitting and receiving a pulse signal at a constant interval at a synchronization timing between first and second communication apparatuses connected through a first optical fiber and a second optical fiber, which are two-core bidirectional, to synchronize time, the time synchronization method comprising: transmitting, from the first communication apparatus, a first pulse signal of a first wavelength and a second pulse signal of a second wavelength different from the first wavelength to the second communication apparatus through the first optical fiber; receiving, at the second communication apparatus, the first pulse signal and the second pulse signal from the first optical fiber; generating a third pulse signal of a wavelength identical to the second wavelength of the second pulse signal when receiving the first pulse signal, and simultaneously transmitting the first pulse signal and the third pulse signal to the first communication apparatus through the second optical fiber; generating a fourth pulse signal of a wavelength identical to the first wavelength of the first pulse signal when receiving the second pulse signal, and simultaneously transmitting the second pulse signal and the fourth pulse signal to the first communication apparatus through the second optical fiber; receiving the first to fourth pulse signals from the second optical fiber at the first communication apparatus; calculating a propagation delay amount in the first optical fiber from a first proportional relationship between a first propagation delay time difference between the first pulse signal and the fourth pulse signal of the identical first wavelength among the received first to fourth pulse signals and a second propagation delay time difference between the first pulse signal of the first wavelength and the third pulse signal of the second wavelength that is different from the first wavelength, and a second proportional relationship between a round-trip delay time in the first and second optical fibers and the propagation delay amount in the first optical fiber; and controlling the first pulse signal and the second pulse signal to be transmitted to the second communication apparatus with a time difference corresponding to a difference between current and last values of the calculated propagation delay amounts such that the difference is zero.

4. The time synchronization method according to claim 3 , further comprising: performing a first control of the first pulse signal and the second pulse signal to be simultaneously transmitted to the first optical fiber at a beginning of transmitting and receiving the first and second pulse signals between the first and second communication apparatuses, and performing a second control of the first pulse signal and the second pulse signal to be transmitted with a time difference corresponding to a difference between the propagation delay amount in the first optical fiber calculated according to the first control, and a predetermined propagation delay amount, serving as a reference, in the first optical fiber such that the difference is zero.